1. Field of the Invention
The present invention relates to wireless communication networks and, more particularly, to baseband recovery in wireless networks, base transceiver stations, and wireless networking devices.
2. Description of the Related Art
Data communication networks may include various computers, servers, nodes, routers, switches, bridges, hubs, proxies, access devices such as Customer Premises Equipment (CPE) and handsets, and other network devices coupled to and configured to pass data to one another. These devices will be referred to herein as “network elements.” Data is communicated through the data communication network by passing encrypted on non-encrypted protocol data units, such as Internet Protocol (IP) packets, encoded and either compressed or non-compressed voice packets, Ethernet Frames, data cells, segments, or other logical associations of bits/bytes of data, between the network elements by utilizing one or more communication links between the devices. A particular protocol data unit may be handled by multiple network elements and cross multiple communication links as it travels between its source and its destination over the network.
In a wireless network, radio frequency (RF) signals are used to transmit data between network elements. Typically, a transmitter will include a RF transmitter that includes a data modulation stage that converts raw data into baseband signals in accordance with the particular wireless communication standard in use on the network. The baseband signals are then modulated by the RF transmitter onto a radio frequency (RF) carrier for transmission on the network. The modulated RF carrier is then amplified and transmitted via an antenna over the air as electromagnetic energy.
Many different modulation schemes have been devised to enable data to be transmitted on wireless networks. One example modulation scheme is commonly referred to as Orthogonal Frequency Division Multiplexing (OFDM). In OFDM, high-speed data signals are divided into tens or hundreds of lower speed signals that are transmitted in parallel over respective frequencies (subcarriers) within a radio frequency (RF) signal. The frequency spectra of the subcarriers may overlap so that the spacing between them is minimized. The subcarriers are also orthogonal to each other so that they are statistically independent and do not create crosstalk or otherwise interfere with each other. In OFDM, each block of data is converted into parallel form and mapped into each subcarrier as frequency domain symbols. To get time domain signals for transmission, an inverse discrete Fourier transform or its fast version, IFFT, is applied to the symbols. One network that uses OFDM as the modulation scheme is commonly referred to as WiMax. WiMax is defined by the IEEE 802.16x suite of protocols. Another emerging standard that has not yet been ratified, but appears likely to use the OFDM modulation scheme is referred to as Long Term Evolution (LTE). Other networking protocols may also use OFDM.
Another example modulation scheme that may be used to modulate the baseband signals for transmission is commonly referred to as Time Division Multiple Access (TDMA). In a TDMA network, the entire frequency spectrum is used to transmit data for a particular channel during a particular time interval. Different channels are allocated different time slots during which data associated with that channel will be transmitted. One example of a wireless network that uses TDMA as the modulation scheme is commonly referred to as Global System for Mobile Communication (GSM), although other wireless communication protocols also use time division to divide the channel into multiple subchannels.
Law enforcement and emergency personnel frequently use push-to-talk radio systems to communicate with each other. One type of communication system that has been implemented for law enforcement and other public safety radio networks is defined by the Project 25 (P25). Project 25 is defined by a suite of protocols developed by the Telecommunications Industry Association (TIA), for example TIA 102-BAAA-A. In the United States, the commercial implementation of TIA 102.BAAA-A is commonly referred to as APCO project 25 or simply APCO 25. In Europe, the implementation of the P25 standard is commonly referred to as Terrestrial Trunked Radio (TETRA).
The P25 Phase 1 specification includes two alternative digital modulation schemes, C4FM and CQPSK. C4FM is a constant-envelope, four-level frequency modulation scheme that operates in 12.5 kHz channels. CQPSK is a compatible differential offset four-level quadrature phase shift keying modulation scheme. The P25 standard provides a symbol transmission rate of 4800 baud using two bits per symbol regardless of whether the channel is modulated using C4FM or CQPSK.
As discussed above, there are many different ways for baseband signals to be RF modulated to be transmitted on a wireless network. The receiver will know what modulation scheme has been used and, hence, will know how to demodulate the signals to recover the baseband signals. However, it is still necessary to synchronize the receiver with the transmitter, so that the receiver is able to sample the received RF signal correctly to recover the baseband signals. In a push to talk wireless communication channel, such as used in an APCO 25 or TETRA network, the receiver must re-synchronize to the transmitter on every transmission burst. Similarly, in a TDMA system, a receiver may only receive symbols during a short period and, hence, must re-synchronize with the transmitter periodically. In GSM, the receiver will need to synchronize with the transmitter on every packet. GSM also provides for synchronization in the middle of the packet.
Synchronization of the receiver with the transmitter enables the receiver to sample the received demodulated symbol stream at the correct frequency and at the correct position within the symbol interval. Ideally, the receiver should sample the received symbol stream at the point of minimum inter-symbol interference, which is commonly referred to as the point of maximum eye opening. The term “timing frequency” will be used herein to refer to how often the receiver should sample the symbol stream, and the term “timing phase” will be used herein to refer to the position within the symbol interval where the receiver should sample the symbol. The process of synchronizing the receiver with the transmitter will be referred to herein as “timing recovery.” Depending on how the receiver is implemented, timing recovery may involve recovery of the timing phase only, or recovery of both the timing frequency and timing phase.
There are multiple ways of synchronizing the receiver and transmitter. One of the common ways of doing this is to allow the receiver to extract the clock signal from the received data signal. While doing this avoids the use of a separate synchronization timing signal, it requires a portion of the data channel to be allocated to timing recovery. For example, at the beginning of a burst of data (or during the transmission of data in the case of GSM), the transmitter may transmit a sequence of known symbols, referred to hereafter as a “training sequence,” that may be used to synchronize the receiver to enable the receiver to learn the symbol phase and correct the timing frequency. Symbols transmitted during the training sequence will be referred to herein as “training symbols.” Since the transmission of a training sequence uses spectrum from the data transmission channel, it is desirable to minimize the amount of spectrum that is used to synchronize the transmitter and receiver. Accordingly, it is common to try to reduce the number of training symbols that must be transmitted during the training sequence to maximize an amount of the data channel that may be used for the transmission of data.
One way to implement timing recovery is to perform a fast Fourier transform on the received signal, look at the frequency components of the signal, and deduce the signal timing from the relative strengths of the frequency components. This type of timing recovery is commonly implemented in WiMax networks and is expected to be used in LTE networks. While this process works well given sufficient processing power, not all handheld devices may be provided with sufficient processing power to implement this method.
Another way to implement timing recovery is to sample the received signal multiple times during each expected symbol interval during the training sequence, and compare the perceived symbols with an expected symbol pattern. Recovery of signal timing using this method is commonly referred to as Maximum Likelihood Estimation (MLE).
When MLE is used to perform timing recovery, it is common to sample the received signal very frequently, on the order of 1000 times per symbol interval, to obtain a fairly high resolution pattern of the received symbols. This representation is compared with an expected symbol representation and the result may be used to adjust a Phase Locked Loop (PLL) at the receiver to adjust the frequency and phase of the receiver. Generally, this process is performed on each training symbol independently and the process is iterated for each training symbol of the training sequence to enable the receiver to incrementally synchronize its frequency and phase with the transmitter.
Sampling the received RF signal very frequently, for example on the order of 1000 times per expected symbol interval, requires significant processing power. To reduce the complexity of the processing circuitry, it would be advantageous to implement maximum likelihood estimation for timing recovery using a much lower symbol sampling rate. However, when a lower sampling rate is used to perform timing recovery, the timing recovery process is more prone to find false lock points and exhibit positive feedback behavior at symbol boundaries. Thus, if the number of samples taken per expected symbol interval is reduced, to reduce the processing required at the transmitter, it becomes necessary to increase the number of training symbols that are required to be transmitted to enable timing recovery to be accurately implemented. Increasing the number of training symbols, of course, impacts the amount of data which may be transmitted on the data channel. Accordingly, it would be advantageous to provide a network, base transceiver station, and mobile station, that would enable the baseband signal timing to be recovered using a relatively low sampling rate to minimize the required processing power of the receiver, while still minimizing the number of training symbols that must be transmitted during the training sequence, to enable the throughput on the wireless channel to be maximized.